The invention is a result of the research made in the field of telescopic jibs provided with a jack for controlling the telescoping thereof. It is already known to use a double jack, comprising two pistons imbricated one in the other, and two piston rods each integral with one of the two pistons, and to couple the two rods and the cylinder to three successive portions of the jib.
The known double jacks present one or more internal connections, each between two of the working chambers that they comprise, which allow combined extensions of the various elements of the jack, which, of course, also allows combined extensions (and retractions) of the various portions of the telescopic jib, with respect to one another.
U.S. Pat. No. 2,616,489 discloses such a jack.
These known arrangements are most often satisfactory for a determined use of the telescopic jib, but do not allow modes of operation other than the one resulting from the above-mentioned internal connections which have been provided.
The rigid nature of the unique mode of operation provided initially is being considered more and more as a drawback by the users, and it is an object of the invention to overcome this drawback